1. Field of the Invention
The present invention relates to an inkjet recording ink, and to an ink cartridge, inkjet recording device, inkjet recording method, and ink recorded matter using the inkjet recording ink.
2. Description of the Background
An inkjet recording system has advantages that it is easy to produce full-color images as a process is simple compared to other recording systems, and gives images of high resolution with a device of a simple structure. As for an ink used for the inkjet recording system, a dye-based ink in which various water-soluble dyes are dissolved in water, or a mixed liquid of water and an organic solvent, has been used. The dye-based ink has however a disadvantage that it has poor light fastness though having excellent vividness of color tone. Therefore, an ink used a pigment that excels in water resistance and weather resistance over the dye has been used.
As a method for dispersing a pigment in an aqueous medium, there has been known a method for using a dispersant such as a surfactant and a water-soluble resin, a method for surface treating a pigment so that a carboxyl group, hydroxyl group, sulfone group, phosphoric acid group, quaternary ammonium or salts thereof is bonded to the surface of the pigment, and a method for coating the pigment with a resin bonded with an ionic group.
Japanese Patent Application Laid-Open (JP-A) No. 2001-81372 discloses an inkjet recording ink in which a pigment is dispersed by polyoxyethylene ether sulfate, as hence having excellent long term storage stability, having vivid color saturation. JP-A No. 2003-327880 discloses, moreover, an ink in which colorant particles prepared by coating inorganic pigment particles with an organic pigment or carbon black are dispersed by an anionic surfactant for the purpose of attaining an ink having excellent ejection stability and storage stability, and hence the ink having excellent ejection stability and the color tone close to that of dye inks.
However, use of a surfactant in a large amount for improving dispersion stability leads to problems such as generation of bubbles, and deterioration in image quality. Therefore, it is difficult to stably disperse a pigment over long period of time. Since inkjet printers have been currently used in various purposes, not only long term storage stability and color tone, but also improvements in fastness and image density of prints are desired.
JP-A No. 2009-149758 discloses a self-dispersible pigment ink containing water-insoluble polymer particles. This ink can form images of high fastness as the pigment is coated with a resin, but the particle diameters are large as coated by the resin. Therefore, the ink has a problem that it has poor ejection stability.
As described above, the pigment dispersed by the surfactant gives prints of poor fastness, and requires the surfactant to attain a stable dispersion state, and in same cases, causes generation of bubbles, and deterioration in image quality. On the other hand, the resin-coated pigment does not require a dispersant, and has excellent abrasion resistance and marker resistance, but cannot easily achieve both desirable image density and ejection stability. Therefore, it is difficult to attain an ink capable of producing prints of high image fastness, while maintaining storage stability and ejection stability.
Considering these situations, it is one of the ideas that a combination of the resin-coated pigment and the surfactant-dispersible pigment is used in an ink. In the case where pigments each dispersed in a different manner are mixed, however, there is a problem that one pigment influences another pigment to cause aggregations, impairing dispersion stability.
JP-A No. 2009-149815 discloses that pigments each dispersed in a different manner, i.e., a surfactant-dispersible pigment and a self-dispersible pigment, are dispersed by a naphthalene-sodium sulfonate condensate, but image fastness of prints formed with this ink is not sufficient, and application of this ink is restricted to a black ink, cannot be applied for color inks.